The invention relates to a microlithography objective, a projection exposure apparatus containing the objective, and a method of manufacturing an integrated circuit using the same.
Using a lithography system operating with wavelengths below 193 nm for imaging structures of below 130 nm resolution has been proposed. In fact, such lithography systems have been suggested for the extreme ultraviolet (EUV) range with wavelengths of xcex=11 nm or xcex=13 nm producing structures of below 100 nm. The resolution of a lithographic system is described by the following equation:
RES=k1xc2x7xcex/NA,
where k1 is a specific parameter of the lithographic process, xcex is the wavelength of the incident light, and NA is the image-side numerical aperture of the system. For example, if one assumes a numerical aperture of 0.2, then the imaging of 50 nm structures with 13 nm radiation requires a process with k1=0.77. With k1=0.64, the imaging of 35 nm structures is possible with 11 nm radiation.
For imaging systems in the EUV region, substantially reflective systems with multilayer coatings are available as optical components. Preferably multiple layers of Mo/Be are used as multilayer coating systems for systems operating at xcex=11 nm, whereas Mo/Si systems are used for xcex=13 mm. With the reflectivity of the multilayer coatings approximating 70%, it is desirable to use as few optical components as possible in applications such as EUV projection objective microlithography to achieve sufficient light intensity. Specifically, to achieve high light intensity and to allow for the correction of imaging errors, systems with six mirrors and a numerical aperture (NA)=0.20 have been used.
The six-mirror systems for microlithography have become known from the publications U.S. Pat. No. 5,686,728, EP 779,528 and U.S. Pat. No. 5,815,310. The projection lithography system according to U.S. Pat. No. 5,686,728 has a projection objective with six mirrors, where each of the reflective mirror surfaces has an aspherical form. The mirrors are arranged along a common optical axis in such a way that an obscuration-free light path is achieved. Since the projection objective known from U.S. Pat. No. 5,686,728 is used only for UV light with a wavelength of 100-300 nm, the mirrors of this projection objective have a very high asphericity of approximately xc2x150 xcexcm as well as very large angles of incidence of approximately 38xc2x0. Even after reducing the numerical aperture to NA=0.2, an asphericity of 25 xcexcm from peak to valley remains, with little reduction in the the angle of incidence. Such asphericities and angles of incidence are not practicable in the EUV region according to the present state of the art because of the higher requirements on surface quality and reflectivity of the mirrors in these latter systems.
Another disadvantage of the objectives disclosed in U.S. Pat. No. 5,686,728, which precludes their use with wavelengths below 100 nm such as the 11 nm and 13 nm wavelengths desirable for EUV microlithography, is the short distance between the wafer and the mirror lying closest to the wafer. This short distance allows only very thin mirrors to be used in the U.S. Pat. No. 5,686,728 apparatus. Because of the extreme stresses on the coatings of the multilayer systems suitable for the 11 nm and 13 nm wavelengths in question, such thin mirrors are very unstable.
A projection objective with six mirrors for use in EUV lithography, even at wavelengths of 13 nm and 11 nm, has become known from EP 779,528. This projection objective also has the disadvantage, however, that at least two of the six mirrors have very high asphericities of 26 and 18.5 xcexcm. Furthermore, in the EP 779,528 arrangement, the optical free working distance between the mirror next to the wafer and the wafer is so small that either instabilities occur or a negative mechanical free working distance is obtained.
Four-mirror projection objectives have become known from the following publications:
U.S. Pat. No. 5,315,629
EP 480,617
U.S. Pat. No. 5,063,586
EP 422,853
In U.S. Pat. No. 5,315,629, a four-mirror projection objective with NA=0.1, 4xc3x97, 31.25xc3x970.5 mm2 is claimed. From EP 480,617, two NA=0.1, 5xc3x97, 25xc3x972 mm2 systems have become known. The system according to U.S. Pat. No. 5,063,586 and EP 422,853 have a rectangular image field of at least 5xc3x975 mm2. These generally decentered systems exhibit very high distortion values. Therefore, the objectives can only be used in steppers with distortion correction on the reticle. However, the high level of distortion makes such objectives impractical at wavelengths below 100 nm.
From U.S. Pat. No. 5,153,898, overall arbitrary three to five-multilayer mirror systems have become known. The disclosed embodiments, however, all describe three-mirror systems with a rectangular field and small numerical aperture (NA less than 0.04). Therefore, the systems described therein can only image structures above 0.25 xcexcm in length. The distortion of most examples lies in the xcexcm range.
Furthermore, reference is made to T. Jewell: xe2x80x9cOptical system design issues in development of projection camera for EUV lithographyxe2x80x9d, Proc. SPIE 2437 (1995) and the citations given there, the entire disclosure of which is incorporated by reference.
Thus, it is desirable to provide a projection objective suitable for lithography with short wavelengths, preferably smaller than 100 nm, which does not have the disadvantages of the state of the art mentioned above, and which has as few optical elements as possible and a sufficiently large numerical aperture.
According to the invention, the short comings of the prior art are overcome by using a projection objective device which has five mirrors. By omitting a mirror from the known six-mirror systemsxe2x80x94according to the five-mirror system of the inventionxe2x80x94one can achieve a transmission which is at least 30% higher at wavelengths in the EUV region, if a reflectivity of the multilayer coating system of 70% is assumed for this radiation. In addition, numerical apertures of NA greater than 0.10 can be realized. The five-mirror objective according to the invention is thus characterized by high resolution, low manufacturing costs and high throughput.
In a first embodiment of the invention, the mirror closest to the wafer is arranged in such a way that the image-side numerical aperture NA is greater than or equal to 0.10. Furthermore, the mirror closest to the wafer is arranged such that (1) the image-side free optical working distance corresponds at least to the used diameter of the mirror closest to the wafer, (2) the image-side optical free working distance is at least the sum of one-third of the used diameter of the mirror closest the wafer and a length which lies between 20 and 30 mm, and/or (3) the image-side optical free working distance is at least 50 mm. Preferably, the optical free working distance is 60 mm.
In a second embodiment of the invention, the image-side numerical aperture NA is greater than or equal to 0.10, the annular field width W at the wafer lies in the region 1.0 mmxe2x89xa6W, and the peak-to-valley deviation, A, of the aspheres is limited with respect to the best-fitting sphere in the useful region on all mirrors, by:
Axe2x89xa624 xcexcmxe2x88x92129 xcexcm(0.20xe2x88x92NA)xe2x88x922.1 xcexcm/mm(2 mmxe2x88x92W).
In a preferred embodiment, the peak-to-valley deviation A of the aspheres on all mirrors is limited by:
Axe2x89xa69 xcexcmxe2x88x9250 xcexcm(0.20xe2x88x92NA)xe2x88x920.4 xcexcm/mm(2 mmxe2x88x92W).
In a third embodiment of the invention, with a numerical aperture in the range NAxe2x89xa70.10 and an image-side width of the annular field in the range Wxe2x89xa71 mm, the angles of incidence, AOI, on all mirrors, relative to the normal of the surface of a given mirror, is limited by:
AOIxe2x89xa622xc2x0xe2x88x922xc2x0(0.20xe2x88x92NA)xe2x88x920.3xc2x0/mm(2 mmxe2x88x92W).
Combinations of the above may also be used according to the invention. For example, in a preferred embodiment, all three conditions are fulfilled, i.e., the free optical working distance is more than 50 mm at NA greater than 0.10 and the peak-to-valley deviation of the aspheres as well as the angles of incidence in the region defined above.
The asphericities herein refer to the peak-to-valley (PV) deviation, A, of the aspherical surfaces with respect to the best fitting sphere in the used area. The aspherical surfaces are approximated in the examples by using a sphere with center on the figure axis vertex of the mirror and which intersects the asphere in the upper and lower endpoint of the used area in the meridian section. The data regarding the angles of incidence always refer to the angle between the incident rays and the normal to the surface at the points of incidence. The largest angle of any incident light ray occurring on any of the mirrors is always given, i.e., the angle of a bundle-limiting ray. The used diameter will be defined here and below as the envelope circle diameter of the used area, which is generally not circular.
Preferably, the wafer-side optical free working distance is 60 mm.
The objective can be used not only in the EUV, but also at other wavelengths, without deviating from the scope of the invention. In any respect, however, to avoid degradation of image quality, especially degradation due to central shading, the mirrors of the projection objectives should be arranged so that the light path is obscuration-free. Furthermore, to provide easy mounting and adjusting of the system, the mirror surfaces should be designed on a surface which shows rotational symmetry to a principal axis (PA). Moreover, to have a compact design with an accessible aperture stop and to establish an obscuration-free path, the projection objective devices are designed to produce an intermediate image. It is preferred that the system include of a first and second subsystem, where a real intermediate image is formed by the first subsystem (with an imaging ratio in the range of xcex2 less than 0), and that the second subsystem images the intermediate image to a real system image in the wafer plane. In such structures, it is possible to have the plane for the object to be imaged located within the structural space of the entire mirror system, including the first, second, third, fourth and fifth mirrors.
In an embodiment of the invention, the aperture stop B lies on the first mirror. Alternatively, the freely accessible aperture stop can lie, both optically and physically, between the second and third mirrors.
In order to be able to make the necessary corrections of the imaging errors in the five-mirror systems, in a preferred embodiment, all five mirrors are aspherical. However, an alternative embodiment whereby at most four mirrors are aspherical, thus simplifying the manufacturing process, can be achieved. Then it is possible to make at least one mirror spherical, preferably the largest mirror.
In order to reach a resolution of at least 50 nm, the design part of the rms wave front part of the system should be at most 0.07 xcex and preferably 0.03 xcex.
Advantageously, in the examples of the invention, the objectives are always designed to be telecentric on the image-side. In projection systems which are operated with a reflection mask, a telecentric beam path on the object-side is not possible without illumination through a beam splitter which reduces transmission strongly such as known, for example, from JP-A-95 28 31 16. Therefore, the chief ray angles on the reticle are chosen so that vignetting-free illumination is possible. Alternatively, in systems with transmission masks, the projection objective can be telecentric on the object-side. Overall, the telecentricity error on the wafer should not exceed 10 mrad and typically is between 5 mrad and 2 mrad, with 2 mrad being preferred. This ensures that changes of the imaging ratio remain within tolerable limits over the focus.
In the embodiments of the invention, the projection objective is divided into two subsystems. The first subsystem is a reducing three-mirror system, preferably with imaging ratio (xcex2) in range of xe2x88x920.5 greater than xcex2 greater than xe2x88x921.0, and the second subsystem is a two-mirror subsystem.
In addition to the projection objective, the invention also provides a projection exposure apparatus, including at least one projection objective. In a first embodiment, the projection exposure apparatus has a reflection mask, while in an alternative embodiment, it has a transmission mask.
Furthermore, in a preferred embodiment, the projection exposure apparatus is an arc-shaped field scanner, and includes an illumination device for illuminating an off-axis arc-shaped field. Advantageously, the secant length of the scan slit is at least 26 mm and that the arc width is greater than 0.5 mm, so that homogenous illumination becomes possible.
The invention will be described below with the aid of drawings as examples.